Traveling wave tubes



April .8, 1958 w. J. DODDS TRAVELING WAVE TUBES Filed Dot. 1, 1951 Wellg b y fi l ids O R N EY United Stat Paten TRAVELING WAVE TUBES Wellesley J. Dodds, Allentown, N. J.',,assignor to Radio Corporation of America, a corporation of Delaware Application October 1, 1951, Serial No. 249,198 Claims. (Cl. 3153.6)

This invention relates to electronic tubes, and particularly totubes of the traveling or growing wave type.

Inthe usual, traveling wave tube, an elongatedlhelical electrode is disposed with itsaxis coincident with 'a rectilinear electron beam extending from :an electron gun to a collector electrode; With such construction, the gain or amplification of the tube is a function-ofthe length of the helical electrode and, consequently, a highgain tube'is physically long; Also, because of the constructional difficulties, such tubes oftemrequire a high-accelerating potential to obtain the necessary initial linear velocity of the electrons.

In accordance with the present invention, which afi ords high gain without need of excessively high accelerating voltage'or excessive length of tube, the usual rectilinear electron "beam is replaced by a substantially axially moving, non-rotating, helical electron beam. In the interactionregiomthe helical beamsweeps over or passes through ahelical electrode structure whose axis substantially. coincides with that of the helical beam and is of suitably'diflerent pitch for the desired interaction between the beam and the helical electrode structure.

. More particularly, in accordance with the invention, the electrons from the gun are first subjected simultaneously to an alternating electric field and an axial magnetic field of such strength as to-produce 'a revolving-pencil type beam and, upon entrance to a second zone, are subjected to "a magnetic field of much greater strength which causes furthermovementof the beam as a helix having .motion substantially of translation only toward the collector electrode.

Inone form of the invention, the electrode structure swept by the helicalbeam comprises a' two-conductor transmission line of helical form and whose pitch is dif-j' fer'ent from the pitch of the beam so thatthere will be favorableinteraction between the'beam 'andthe' circuit constituted by the line, l 'f v The inventionfurther resides in methods and arrangementshaving features of novelty hereinafter described and: claimed. j i 1 For a more detailed understanding of the invention and-forillustration of embodiments thereof, reference is made to the'accompanying drawings in which:

'Fi grl schematically illustrates the new type of travel ing wave tube and its method of operation;

Fig. 2 is an axial sectional view of one form of the new traveling wave tube and its circuit connections; and

Fig. 3 is an axial sectional view-of a modificationof the two-conductor helical output structure of Fig.2. 7

In accordance with the present invention,'an axial constant magnetic fieldis combined with a transverse alternatingclectrical field in a first region to produce a revolving-pencil beam of spirally-traveling electrons. Then the electrons move into a second region having an-axial magnetic field-of much greater strength. The strong magnetic field constrains the electrons to motion in helical paths" parallel to the magnetic field of such small radii'that for 1 2,830,221 Pat nte AP.- 9

practical purposes the path of each electron may be considered straight. Since-the electrons of the rotating path to a collector electrode 14, forming'a'straight line beam indicated by a dot-dash-line S, Fig. 2. Such straight line beam is utilized in prior'types' of traveling wave tubes and often requires,'for high gain, the application of high accelerating .voltage to the electrodes 13 and 14, or

equivalent, and also requires a tube-which is physica'llylong.

In the new typeof traveling or growing wave tube, the electrons from gun 10 are subjected simultaneously in zone #1 to an alternating electric field E transverse to axis S and to a static, coaxial'magnetic field H These two fields cause electron beam R to revolve about axis S in zone #l, generating a cone C whose apex is directed toward the gun 10'and whosebase B is'presented toward the collector electrode 14; More precisely explained, as the electric field E deflects 'the electrons entering zone #1, they begin to move across the axial magnetic field H This motion transverse'to the magnetic field causes the'electrons to be deflected-at right angles to the radial motion. The magnetic field strength H is so chosen or adjusted that I i is equal to 21rf where e and mare the charge andmass, respectively, of the a electron, and his the-frequency of the electric field E Underthese conditions all the elec-' trons tend to move in circular paths with the same angular velocity and in-phase with. the electric, field. Hence, the electrons are continuously accelerated, radially, moveing in spiral paths of increasing radii. At any instant, all the, electrons lie on a straight line, which revolves about the axis S to generate the cone C. The fields for'efiecting such revolution of thebearri are produced by application of a signal voltage of frequency f to the deflection:

plates 15, 15 disposed within zone #1 on opposite sides 20, l951, to C. L. Cuccia.

.As the beam passes from zone #1 into zone, #2, his; subjected to asecond coaxial magneticfield H of field strength much greater than that of field H in zone #11;

p In this second zone, the electronbeam assumes the form of a helical beam ,T of diametercorresponding with the,

base .of cone C. This helical electronbeamidoesmot. rotatebut has motion substantially of translation only; toward the collector electrode .14. As each .electromofz .beamfR, traveling in a spiral path in zone tal -enters? {zone #2, it is constrained .by the strong rnagneticfield;

; may be considered as substantially a straight line to traversea helical path parallel to the magneticjfleld;

of much'smallerradius r determinedby the relation;

II36' V p where v is the tangential velocity entering zo and H is the strong magneticfield strengthinzonejt i; This radius r can be made so small that the h 1' al pgthi,

In its passage through zone #2, the helical beam sweeps across a helical conductive ,structure, shown in Fig. l, for example, as a two-conductor helical transmission line 16. The relative pitch of the axiallyxmoving helical beam may be so'chose'n that the. points X, where the beam at some instant 'is sweeping the space between the conductors of the line 16, are one wavelength apart on the line. 1 Under suchlcircumstance, there will be induced in the coiled transmission line voltages which are of equal amplitude and like phase at points one wavelength apart along the line. The axial motion of the helical beam with respect to thestationarycoiled transmission line causes the points X progressively to slide along the helical output structure at a rate determined by the pitch of the output helix 16, the'pitch of the helical beam and the velocity of translation. of the beam along the axis S. These parameters may be adjusted so that the .velocity of points X along the convolutions of the transmission line 16 ,with' respect to'the characteristic velocity of a wave moving down the line producesimaxirnurn. signal output. 7

It can be shown that if the helical beam and the line 16 havethe same helix sense, the points X will move forward, in the same direction as the beam, if the beam pitch is less than the line pitch. This condition is shown in Fig. 1, where, for. the sake .of clarity of illustration only, the beam T is shown as having a pitch one-half of the pitch of the line 16. e

, If the beam and the line have opposite helix sense, the

points X will move forward whether the beam pitch is greater or less than the line pitch. It will be understood that the helix sense of the helical beam can be reversed merely by reversing the direction of the axial magnetic 7 terminals for connection to a signal source and output field. When the beam and line have the same helix sense I the speed of movement of the points X increases as the beam and line pitches are made more nearly equal. Therefore, the axial velocity of the beam may be adjusted to produce any desired interaction speed between the beam and the line. y

Because of the interaction between the'beam and "the signal wave on the transmission line, the spacing between the electrons does not remain fixed in zone #2. There is repeated alternate bunchingand separation of the electrons, the bunching becoming more and more pronounced as the collector is approached with consequent increase or growth of'the wave; traveling along the transmission line. This'amplification phenomenon, in itself, is substantially that which occurs in traveling-wave tubes having an electron beam flowing rectilinearly'through, and in coupling relation with, a helix or other traveling-wave circuit.

One embodiment of a tube utilizing the aforesaid novel principles is shown in-Fig. 2, in which elements corresponding with those of Fig. 1 are identified by the same reference characters.

The electrons in passing through zone #2 eventually giveup a substantial part of their energy to the transmission line 16, with resultant amplification of the traveling wave on the transmission line. For collection of the electrons after passage through zone #2, the collector electrode 14 is suitably positively biased at a relatively low positive potential by a direct-current voltage. source,

exemplified by battery ;17. I For imparting tot he elec--' trons the required initial velocity, the accelerating electrodel3, orequivalent, is suitably positively biased as' by direct-current voltage source, exemplified by battery 18. With suchyan arrangement, as compared with traveling wave tubes of prior type, this accelerating potential for a given pitch'of the helix is much less. The means for producing the fields H and H in zones #1 and #2 may be coils 19A and 19B fitting over the envelope20'of the tube, or alternatively, either may be a circular permanent magnet magnetized to produce a lope 20, as shown in Fig. 2, to provide input and output load, respectively.

In the above discussion, the effects on the beam due to the transition from the first magnetic field to the.

second magnetic field have been ignored. In a practical embodiment of the invention, this transition will necessarily introduce at least some components of magnetic field transverse to the axis of the tube. Such transverse components will cause a reduction in the axial component of beam velocity in zone #2 which must be talten into consideration in determining the pitch of the output transmission line 16.

In the form shownin Fig. 2, the helical output structure is a two-wire transmission line 16 formed of parallel wires suitably supported internally of the envelope of the tube, Alternatively, as shown in Fig. 3, the output structure. may be a tubular metallic, non-magnetic member 21, designed either to fit within the envelope of the tube or to form part of it. In either case, the cylindrical memberis internally grooved as indicated at 22 to provide the equivalent of the two-wire helical transmission line of Fig. 2. 1

.'..It shall be understood that the invention isnot limited to the. specific embodiments illustrated and that other forms of traveling. wave tubes in which the electron beam traverses the, drift space as a helix are within the scope of the appended claims. i

What is claimed is:

1. A traveling wave tube comprising an electron gun and a collector electrode spaced from each other and defining a beam path therebetween, field-producingmeans forsubjecting the electron beam in its passage from said gun toward said collector electrode to electric and magneticfields to produce a helical beam of electrons having substantially axial translational movement only, and helical-conductive structure-swept by said helical beam, said field-producing means comprising deflection plates on opposite sides of said beam path to which an alternating signal is applied, and in which the pitch of the beam is different from that of said helical conductive structure.

netic fields to produce a helical beam of electrons having.

substantially axial translational movement only, and helical conductive structure swept by said helical beam,

said field-producing means comprising means for applying to the electron beam an alternating electric field and a constant magnetic field respectively transverse and parallel to said beam path in a first zone adjacent to said gun to elfect revolution of the beam about said path in a conical path whose base is presented toward the collector electrode, and in which the field-producing means provides a'magnetic field of much greater field strength parallel to said beam path in a subsequent zone for producing said helical beam. v 7 i p 3. A traveling wave tube as in claim 2 in which said helical conductive structure is a two-conductor transmission line coiled to form a helix coaxial with said helical beam.

4. A traveling wave tube as in claim 2 in which said helical conductive structure is of solid construction internally grooved to form ahelix.

5. A traveling wave tube as in claim 2 in which said helical beam and said'helical conductive structure have helical beam and said helical conductive structure have opposite helix sense.

7. A traveling wavetubecomprising an elongated helical cohductivestructure, and means for producing a helical beam of electrons. having substantially transla tional movement only along the axis of said helical conductive structure; said means including: means spaced from said structure for producing a revolving beam Of electrons which traces a circular trace coaxial with said structure in a transverse plane located adjacent to one end thereof; and means for subjecting said revolving beam, in a region containing said structure, to an axial constant magnetic field of sufficient field strength to cause the individual electrons of said beam to follow paths in said zone which are substantially straight and parallel to said axis.

8. A traveling wave tube comprising an elongated helical conductive structure, and means for producing a helical beam of electrons having substantially translational movement only along the axis of said helical conductive structure; said means including: electron gun means, spaced axially from said structure, for projecting an electron beam toward said structure along an initial path parallel to said axis; means for subjecting said beam in a first zone adjacent to said gun means to a transverse alternating electric field and a first axial constant magnetic field of predetermined field strength, to cause the electrons of said beam to spiral about said axis; and means for subjecting said beam in a second zone, substantially contiguous to said first zone and containing said structure, to a second axial constant magnetic field of much greater field strength than said first magnetic field, to cause said spiralling electrons to move in said second zone along paths which are substantially straight and parallel to said axis.

9. A traveling wave tube as in claim 8, wherein said means for subjecting said beam to a transverse alternat- 6 ing electric field comprises a pair of deflection plates on opposite sides of the beam path between said electron gun means and said structure.

10. The combination of a traveling wave tube as in claim 9 with a source of alternating voltage of predetermined frequency, wherein the field strength of said first magnetic field is equal to said frequency multiplied by 21rm/e, where m and e are the mass and charge, respectively, of an electron.

References Cited in the file of this patent UNITED STATES PATENTS 1,991,282 Kohl Feb. 12, 1935 2,122,538 Potter July 5, 1938 2,241,976 Blewett et a1. May 13, 1941 2,295,315 Wolif Sept. 8, 1942 2,376,707 McCoy May 22, 1945 2,435,601 Ramo Feb. 10, 1948 2,439,401 Smith Apr. 13, 1948 2,542,797 Cuccia Feb. 20, 1951 2,565,357 Donal Aug. 21, 1951 2,575,383 Field Nov. 20, 1951 2,578,434 Lindenblad Dec. 11, 1951 2,579,654 Derby Dec. 25, 1951 2,584,308 Tiley Feb. 5, 1952 2,591,350 Gorn Apr. 1, 1952 2,598,301 Rajchman May 27, 1952 2,608,668 Hines Aug. 26, 1952 FOREIGN PATENTS 993,156 France July 18, 1951 

